1. Field of the Invention
This invention relates to bipolar transistors and processes for making bipolar transistors and in particular relates to a self-aligned polysilicon, polysilicide, or silicide base contact in a bipolar junction transistor.
2. Description of Related Art
Bipolar transistors are common integrated circuit elements having many uses. FIG. 1 shows an example of a prior art NPN bipolar transistor 100 having a base 102, an emitter 103, and a collector 104 formed in a silicon substrate 101. Base 102 has P type doping which is the opposite doping type from the N type doping used in emitter 103, collector 104, and substrate 101. Accordingly, junctions are formed between base 102 and emitter 103 and between base 102 and substrate 101. Metal contacts 106, 107, and 108 respectively contact base 102, emitter 103, and collector 104 through openings 109 in an insulating layer 105 so that bipolar transistor 100 can be connected to other circuit elements. In forward active mode, voltages applied to emitter 103 and collector 104 tend to forward bias the junction between base 102 and emitter 103 and reverse bias the junction between base 102 and substrate 101, and as is well known, a base current through contact 106 controls much larger emitter and collector currents through contacts 107 and 108 respectively.
Several manufacturing constraints limit the minimum size of transistor 100. For example, lithography and etching techniques limit the minimum size of openings 109. Accordingly, active regions in substrate 101 such as emitter 103 must be larger than the minimum opening size, otherwise the emitter contact 107 would also could contact base 102. Lithography and etching techniques also limit the separations between metal contacts 106 to 108 (and between base 102, emitter 103, and collector 104). The separations must be sufficient to permit etching which electrically isolates metal contacts 106 to 108 from each other. Further, since metal contacts 106 to 108 and openings 109 are formed using different masks, the separation between openings 109 (and between base 102, emitter 103, and collector 104) must be large enough that the expected misalignment between the mask which defines openings 109 and the mask which defines contacts 106 to 108 still provides working contacts.
Accordingly, structures and manufacturing processes are needed which reduce the size of active regions in bipolar transistors.